1. Technical Field
The invention relates to a piezoelectric actuator, a drive control method of piezoelectric actuator, and an electronic device
2. Related Art
In view of excellent conversion efficiency from electric energy to mechanical energy and high responsivity of piezoelectric element, a piezoelectric actuator that drives an object by transmitting a vibration of a piezoelectric element has been recently developed.
One of known piezoelectric actuators employs a ring piezoelectric element (Document 1: Japanese Patent Laid-Open Publication No. 2005-86884). Since such piezoelectric actuator has greater electromechanical coupling coefficient of piezoelectric element than a piezoelectric element of a shape having longitudinal direction (e.g. rectangle), large output can be obtained for the dimension thereof.
In order to drive the piezoelectric actuator, drive electrodes are provided on both sides of a bisector along the diameter of the piezoelectric element, and drive signal is supplied to the drive electrodes by a predetermined phase difference to resonate the piezoelectric actuator. At this time, respiratory vibration (each area provided with each drive electrode reciprocates from inside to outside and from outside to inside along radial direction of the piezoelectric element) and bending vibration (the areas on both sides of the bisector are eccentrically oscillated in a direction orthogonal to the bisector on account of the phase difference between the respiratory vibrations of the respective areas) are excited on the piezoelectric actuator.
Since the related-art ring-shaped piezoelectric actuator as shown in Document 1 is driven without detecting the vibration generated on the piezoelectric actuator, resonation cannot be accurately recognized and efficient drive of the actuator have been difficult. Accordingly, it is conceivable that a detection electrode for detecting the vibration is provided on the piezoelectric element and the drive frequency is controlled based on the detected vibrating condition. In controlling the drive frequency, drive action may be unstabilized when the drive frequency is directly changed, so the phase difference of the vibration signal relative to the drive signal is fed back to alter the drive frequency in a tracking manner. In other words, the phase difference of the vibration signal relative to the drive signal is an index for controlling the drive of the actuator, and the drive control becomes difficult if the phase difference is not monotonic increase or decrease because the direction for increasing or decreasing the drive frequency cannot be determined. However, the phase difference may not exhibit monotonic increase or decrease according to the position of the detection electrode. Because of such difficulties, related-art ring-shaped piezoelectric actuator has no detection electrode. Accordingly, since the drive frequency is not appropriately controlled, the piezoelectric actuator may be excessively excited to cause damage on the actuator.